During the operation of ungrounded sources or loads, so-called standing waves can occur if, in the line leading from the source to the load, e.g., in a coaxial line, the outgoing current in the inner conductor of the coaxial line does not exactly correspond to the return current in the outer conductor of the coaxial line. Therefore, a non-shielded current arises on the outer conductor and is being referred to as a standing wave or as a common-mode wave.
Such standing waves fill the free space surrounding the line with an electromagnetic field and, therefore, depending on the area of application of the line, may bring about disadvantageous and disturbing effects, e.g., undesired oscillations in amplifier systems or dangerous effects on the immediate surroundings such as skin burns on patients in a magnetic resonance installation.
For standing wave suppression, various so-called standing wave traps have been proposed, such as in EP0337204A1. In the conventional art, in order to realize a standing wave trap, locally a part of the coaxial line is rolled up to form an inductance. The shield of the coaxial line is bridged at the beginning and end of the resultant coil with a capacitance, such that the capacitance together with the inductance forms a parallel resonant circuit at the operating frequency of the line. On account of the finite coil quality factor, the loss resistance, Rp, in parallel with the coil remains as a residual path for the standing wave. It holds true here that the standing wave suppression becomes better with higher Rp=ωLQ. In this case, ω is the angular frequency, L, is the inductance of the coil and Q is the coil quality factor. Accordingly, a good suppression requires a high coil quality factor Q, achievable via a large volume of the coil, and/or a high inductance, L, achievable via a high number of turns and again via a large coil volume. However, the measures for enlarging the coil volume and/or for increasing the number of turns are accompanied by a corresponding lengthening of the coaxial cable rolled up to form the coil, and thus increasing the damping of the useful signal in the cable, which is also referred to as push-pull signal.
Furthermore, a disadvantageous effect arises from the fact that such standing wave traps are special components which generally are expensive and are poorly handleable in particular on account of the spatial dimensions. Thus, conventional standing wave traps also cannot be fitted as an SMD (SMD: “surface-mounted device”).
As an alternative to the standing wave traps described for suppressing standing waves, the so-called Boucherot bridge, may be used. As used herein, the term “Boucherot bridge” encompasses its plain and ordinary meaning, including, but not limited to a bridge which consists of two identical inductances (L1, L2 where L1=L2) and capacitances (C1, C2 where C1=C2). Although cost-effective and possible to fit as an SMD, the inductance value is fixedly predefined by calculation for complying with the characteristic impedance for the useful signal. This inductance value is relatively low. With regard to the common-mode suppression, only half the inductance of the Boucherot bridge actually takes effect, with the result that the suppression effect of the bridge is ultimately insufficient in most cases. Even a cascading configuration of a multiple Boucherot bridges does not lead to a significant improvement. The corresponding individual suppression resistances of the bridges are in series for the common-mode wave, and so the resistances are added together. If e.g., two bridges having identical suppression resistances are used, then the total resistance corresponds to double the suppression resistance. The resulting total resistance is comparable, in principle, with the Rp mentioned above. Since Rp is generally very much greater than the reference impedance (generally 50Ω), the example demonstrated yields only 6 dB more suppression for a cascade configured with two identical Boucherot bridges (2-fold cascade), 9.5 dB for a 3-fold cascade, 12 dB for a 4-fold cascade, etc.